The symbiotic relationship between the nitrogen-fixing Sinorhizobium meliloti and its legume host (Medicago sativa) is the result of an intricate signaling network between the host and the symbiont. Quorum sensing, a process that involves the cell density-dependent regulation of gene expression, has been shown to be an integral part of this complex association. It is increasingly clear that many important bacterial activities essential for the effective colonization and pathogenesis of animal or plant hosts are regulated in a population density-dependent manner. This process involves the release and detection of small extra-cellular signal molecules termed autoinducers. The best characterized of these autoinducers are acylated homoserine lactones (AHLs), mostly found in Gram-negative organisms. The overall goal of this research is to determine the role that quorum-sensing plays in the well-characterized relationship between S. meliloti and its host. As a result of our recent work, this symbiotic association is rapidly becoming a paradigm for the study of quorum sensing. S. meliloti possesses a complex series of linked quorum-sensing systems. We have characterized three of those systems (Mel, Sin and Tra) and established that they play a part in nodulation, exopolysaccharide production and conjugal plasmid transfer. The Sin system produces the longest AHLs detected to date. Our work has determined that one of these long-chain AHLs plays a key role in regulating the production of the symbiotically important exopolysaccharide II. Finally, the Sin quorum-sensing system affects the timing and number of nodules it induces on its plant host. The first aim of this proposal is to determine how the multiple quorum-sensing systems of S. meliloti are regulated and coordinated. Second, we will investigate how the highly hydrophobic AHLs produced by the Sin system are transported across biological membranes. Third, this project seeks to identify the genes controlled by the Sin quorum sensing system and determine their function in the S. meliloti/alfalfa symbiosis. This study will provide a better understanding of the symbiotic/pathogenic relationships between prokaryotes and their eukaryotic hosts and might provide tools for the control and manipulation of such relationships. [unreadable] [unreadable]